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Understanding Microgrids

Hurricanes have severely damaged transmission and distribution lines and caused millions of people to go without electricity for days.

Hurricane Maria wiped out the transmission and distribution lines in Puerto Rico. Superstorm Sandy did the same in New Jersey and New York states.

Proponents of wind and solar have called for replacing the existing grid with distributed generation and microgrids. They claim microgrids will prevent the outages and blackouts caused by storms that damage existing transmission and distribution lines.

For example, Dr. Jennie Stephens, the dean’s professor of sustainability science and policy at Northeastern University, wrote an article for the Wall Street Journal (WSJ) calling for the use of distributed generation and microgrids.

But what are microgrids?

Quoting from my earlier article on Puerto Rico:

“Essentially, a micro-grid is a self contained island within which there is power generation, distribution and storage. While self contained, its designed to operate as part of a larger grid.”

And how would a microgrid really operate?

The Ultimate Microgrid

The ultimate microgrid would consist of a single home with rooftop PV solar for generating electricity and storage, with this “home” microgrid connected to the surrounding grid.

Assuming this “home” microgrid had been located in New Jersey when superstorm Sandy rolled ashore in 2012, we can postulate what would have happened.

The surrounding grid would have been severely damaged and unable to supply the home, i.e., microgrid, with electricity.

The home with two Tesla Powerwall-2 batteries would have enough electricity stored in the batteries to allow the home to continue to operate during the storm and for about 12 hours afterwards, at which point the “home”microgrid would stop operating, i.e., go dead.

If cloudiness persisted for a few days the microgrid would not function and the home would remain without electricity until the sun started to shine. Of course the home could have installed enough storage to last for several days, but there would be a cost associated with that additional storage.

Here is a recap of the investment associated with the above microgrid, which would be capable of supplying electricity for two days. The PV roof top system is replaced every twenty years, while the batteries are replaced every ten years.

Item

Cost for first 20 years

PV rooftop solar system

$46,000

Two Powerwall-2 batteries for 24 hour supply

$28,000

Two Powerwall-2 batteries for additional 24 hour supply

$28,000

Total

$102,000

Note: Assumes roof area provides sufficient space for PV array to face due south. Any deviation results in additional costs.

PV solar system is 50% oversized to permit recharging of batteries when external grid is down.

PV rooftop solar costs from Nothing to Fear.

Battery costs are from the Tesla website.

These costs raise the question whether home owners are willing to pay $5,000 per year for electricity that now costs them around $2,000. (Using New Jersey’s average rate of 13 cents per kWh)

If the external grid’s transmission and distribution lines aren’t repaired within 24 hours after the hurricane, the microgrid would be down and unable to supply electricity unless the sun was shining. If there were more cloudy days the “home” microgrid would remain down.

The above house-centric microgrid cannot operate in isolation from the main grid, with the home going off grid, since there will be periods where cloudiness extends for several days.

During periods of cloudiness, electricity must be available from the main grid to power the home and allow for the recharging of batteries. If the main grid is not available there would be a need for considerably more storage, or adding power generation at the home with natural gas, if available … Or being willing to do without electricity.

Larger Microgrid

But what about a larger microgrid extending over several counties and a larger area?

In this alternate scenario with a larger microgrid, there would be a centrally located distributed generation source, which could be solar, natural gas, or wind, with distribution lines radiating from the power generation site to the homes and businesses within the microgrid.

When superstorm Sandy or hurricane Maria arrive, or similar storms with new names, the distribution lines within the microgrid will be leveled, as will the transmission and distribution lines from the centralized grid within which the microgrid operates.

In this scenario, the homes and businesses would be without power, the same as if there were no microgrid.

There might be instances where the external transmission and distribution lines go down while the microgrid continues to function, but not many. The Northeast blackout in 2003 caused by an overloaded transmission line might be such an example.

Photos by D. Dears

Causes of outages

According to the WSJ section in which Dr. Stephens’ article appeared, 33% of outages were caused by storms and trees, i.e., the scenarios where distributed generation and a microgrid would provide minimum additional reliability.

It’s almost always the failure of transmission or distribution lines that cause an outage. Rarely is an outage caused by the sudden shutdown of a coal-fired, natural gas or nuclear power plant.

Additionally, here is a breakdown of the other causes of outages, as itemized in the WSJ:

Animals caused 4% of outages. A microgrid won’t prevent an outage when a squirrel touches the power-line connected to a bushing of a distribution transformer causing a short circuit where the line is taken out of service by a cutout or other device.

Vehicle accidents, which caused 12% of the outages, wouldn’t be lessened by a microgrid.

Planned outages accounted for 5% of all outages, so microgrids wouldn’t be spared from these outages.

Faulty equipment or human error caused 24% of the outages, and microgrids would be equally susceptible to these same causes.

These caused 79% of all outages, while the cause of remaining outages was unknown.

Contrary to the proponents of wind and solar, distributed generation and microgrids are not a panacea for preventing outages.

Proponents of microgids that I have seen believe that a microgrid would have its own power source, such as natural gas generator or a fuel cell. In other words, they would not totally rely on solar/wind and batteries. They want to operate independently of the grid, or in some cases, they want to operate independently unless they want to use the grid for back up. Obviously there is a price for this kind of redundancy.

Yes. Proposals have usually called for a distributed generation supply within the microgrid. The generation source could include any type of power generation, i.e., natural gas, wind, solar, etc. I had tried to make that clear.
At the same time, proposals have been made calling solely for wind and solar as the power generation sources so as to eliminate CO2 emissions.

I am confused. In the body of your article you state two Tesla batteries provide 12 hours, in the table it is 24 hours. Also, you end your article by stating proponents of microgrids claim they would be a panacea for “preventing outages.” I think that should be “living through outages.”

With respect to the table. The text says that two batteries will last for the duration of the storm plus 12 hours. The table assumes the storm lasts for 12 hours, which when added to the additional 12 hours after the storm accounts for the 24 hour day.
With respect to outages. The proponents of microgrids have said that microgrids will prevent outages, but they are wrong in that regard because the outages are predominantly from downed power lines. It’s also true that a home, as described in the text, could allow someone to live through an outage, with the caveat that they may run out of electricity sometime after the storm is over.
Thanks for the questions.